Fitting Digital Tacho

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Fitting Digital Tacho

Postby MacL » Thu Apr 09, 2020 11:21 am

Can anyone who has fitted a Caerbont Auto "Smiths" 12V digital tacho to a 6V Pos earth system please advise me how to wire it up. I've got a 6/12V stepper "Gismo" to provide the 12v and a plug lead sender unit for the tacho RPM signal. I connected up according to the instructions for Pos earth that came with the Tacho and used 12v and earth lines independent from all other connections. The tacho does not switch on so I obviously have not understood what their instruction terms mean. The 6/12v stepper and RPM signal unit are supplied /labeled for Neg earth modern systems so I have reversed +/- connections accordingly. A wiring schematic/diagram [interpretation of Caerbont's instructions] would be really helpful.

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Re: Fitting Digital Tacho

Postby 56G80S » Thu Apr 09, 2020 3:30 pm

IKBA about electrics but went on the website ( https://www.caigauge.com/documents/prod ... 37-439.pdf ) and the first thing it says is "use on NEGATIVE earth vehicles only". You don't say what model your are fitting it to or that it is an AMC machine. But if it's a dynamo model, unless you are using an older solid state regulator it's fairly straightforward to change over to negative earth and then it's problem solved?

Running both +ve and -ve on the same machine seems like asking for problems unless you are absolutely sure about insulation.

Someone will come along who knows better.

Johnny B

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Re: Fitting Digital Tacho

Postby MacL » Thu Apr 09, 2020 5:25 pm

Hi Johnny, Thanks for reply. They supply Tacho with wiring for either Pos or Neg earth now. Machine is 62 G12CSR Mag/Alt 6v Pos earth. The instructions assume a 12v batt pos earth. Yes - am sure someone has been down this route before. Regards Mike.

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Re: Fitting Digital Tacho

Postby Reynard24 » Fri Apr 10, 2020 9:14 am

Hi Mike
If I read your post correctly then you have simply reversed the connections on the stepper and pick up units - this won’t work and more importantly you have most probably destroyed any polarity sensitive devices in the units and possibly also voided the warranty.
The best way to overcome all the problems you are experiencing is to convert the bike to 12v negative earth which is very straightforward but you may be able to power the stepper separately, if not destroyed, by using a small 6v battery connected to only that unit and wired pos-pos, neg - neg but this may still lead to problems if the pick up for example needs to be earthed to the bike.
In normal times I would say come along to the Fenrunners meeting on Monday but you can give me a ring on the Fenrunners contact number and I’ll try to help you over the phone. James

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Re: Fitting Digital Tacho

Postby 56G80S » Fri Apr 10, 2020 12:26 pm

OK, apologies, I thought the tacho gizmo was the wrong polarity. It's just the step up transormer then. What about using a conversion to 12v? I'm sure I've read about that being done with alternators by changing the wires leading from some "6v" alternators, probably not all.

Or conversely, and I know this sounds facetious but isn't meant to be, are there no step ups that match the polarity?

Good luck.

Johnny B

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Re: Fitting Digital Tacho

Postby MacL » Fri Apr 10, 2020 1:33 pm

Hi, Johnny and James, thanks for the input. I think you are both on the right line with regards volts stepper and signal unit. They both came without fitting instructions. The stepper wires[3] were labeled +/- /0 volt. I found a wiring diag for the signal sender on the net that showed colour coded wiring and from it identified the signal,and, Pos and neg leads. The wiring diag for the sender was for a modern neg earth installation;hence I reversed the +/- wires. I think I went wrong doing the same for the 6/12 v stepper - plus maybe not understanding what 0v connection is. James thanks especially for further offer of aid. With luck most of these Gismos have built in reverse current protection these days . w'ell see. I will get out the AVO and try to get head around this before thinking of doing anything like going 12V neg. Regards and thanks Mike.

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Re: Fitting Digital Tacho

Postby MacL » Tue Apr 28, 2020 11:31 am

Tacho wiring.pdf

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Re: Fitting Digital Tacho

Postby MacL » Tue Apr 28, 2020 11:57 am

MacL wrote:
Tacho wiring.pdf


For info. This is what I am going to do. The trouble was following the Cearbont instructions which were for fitting to a 12Volt main system. I grounded the Tacho earth wires to the bike's earth when they should have been connected to the 6/!2 Volt converters ground. Note :- You need a switch to select power on to the volt converter to isolate it from the bike's Batt or else the converter would draw power when not required.
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